Australian Antarctic Division: Leading Australia’s Antarctic Program

Light Detection and Ranging (LIDAR)

Aurora and LIDAR laser beam (Photo: Rob 'Angry' Cullen)

The LIDAR (Light Detection and Ranging) instrument was used for the study of the middle atmosphere above Davis station from 2001 to 2012. It is currently being re-tasked for investigations of the lower atmosphere.

It is clear that human activity has altered Earth's atmosphere. For example:

the release of chlorofluorocarbons has caused significant thinning of the protective ozone layer, particularly in the polar regions;

increasing levels of human-made greenhouse gases are raising temperatures in the lower atmosphere.

Modelling and observations suggests that the middle atmosphere (10–100 km altitude) is cooling in response to rising greenhouse gas concentrations and ozone loss.

It is still unclear how these changes are influencing the general structure of the atmosphere. However, there are strong indications that changes to the middle atmosphere, in particular the stratosphere (10-50 km), are affecting Australia’s surface weather.

Atmospheric scientists from the Australian Antarctic Division and the University of Adelaide have developed a complex Light Detection and Ranging (LIDAR) instrument that has been used in the detailed study of the atmosphere above Davis station.

Light detection and ranging is a remote sensing technique that is the optical equivalent of radar (radio detection and ranging). A LIDAR instrument obtains information on the location and properties of distant objects by illuminating them with a beam of light (usually from a pulsed laser) and measuring the 'time-of-flight' and other characteristics of the scattered light.

The Davis LIDAR has been used to investigate the long-term climate and characteristics of the Antarctic atmosphere in the study of global climate change, and particular phenomena such as polar stratospheric clouds, polar mesospheric clouds, meteoritic dust, and smoke from the 2009 Australian fires. The LIDAR provides measurements of atmospheric density and temperature as a function of height and various properties of clouds and aerosols along the path of the laser beam. The maximum altitude range is typically between five kilometres and 65 km during daytime, and up to 100 km at night. The measurements are collected with a vertical resolution of 18 metres and an integration time of 60 seconds.

The light beam transmitted by the LIDAR comes from a pulsed solid state (Nd:YAG) laser at a wavelength of 532 nm (in the green part of the optical spectrum). The average power of the laser is 30 Watts; each laser pulse comprises about 3 billion times more photons than the output from a standard electric light globe. About 25% of the laser light transmitted into the sky is scattered or absorbed within the atmosphere – the rest continues out into space.

The laser light backscattered from the atmosphere is collected by two telescopes with apertures of 280 mm. The received light can be analysed with a scannable Fabry-Perot Spectrometer, which is a crucial component for daytime measurements that isolates the laser light from the bright background skylight. Two shutters spinning at 400 revolutions per second are used to accept light scattered from certain altitude ranges, thereby avoiding overwhelmingly strong signals returned from clouds. Use is made of the scattering properties of light to infer the concentration of molecular nitrogen through the atmosphere and the presence of water droplets and ice crystals in clouds.

The accuracy of the temperature measurements depends on altitude, but is typically 1–2 degrees Celsius at an altitude of 30 km over an integration time of 30 minutes (as determined by comparison with in-situ measurements from balloons).